Serial number entry after posting delivery doc......

hi experts....

we have a requirement in which in the standard sales cycle of Sales order->delivery document posting ( PGI)-> invoice billing the client wants that serial number entry (which is done at the time of PGI) should be done after invoicing(at the end of the day) because as per the business practice of the client, its not practically feasible for the client to enter serial numbers at the time of selling the finished material. The client manually records the serial numbers on the invoice copy and enters the serial numbers for the respective invoices at the end of the day.

Now while testing the client scenario we found that once PGI & invoicing has been done without entering the serial number for the material, the system does not accept any serial number assignment for that material after the completion of sales cycle.

So is there any solution in which serial number can be entered for a material whose PGI and invoicing has been completed?